The long range goal of these experiments is to determine the function of the immune response (Ir) genes in the regulation of the immune response and in the control of immunocompetent cell interactions. At present the main areas of investigation include: 1) serological and genetic studies of the antigens of the guinea pig major histocompatibility complex. 2) Physico-chemical characterization of immune response associated (Ia) antigens. 3) The role of the Ia antigens and Ir gene products in macrophage-thymus derived lymphocyte interactions.